1. Field of the Invention
The present invention relates to communications and, more particularly, to code division multiple access techniques.
2. Prior Art
Referring now to FIG. 2 there is shown a block diagram of a direct sequence coded spread spectrum system. As shown in the diagram a carrier signal generated by carrier generator 80 is modulated at mixer 98 by data 82. The data modulation operates at a rate determined by code clock divided by spreading gain G 84. The modulated signal is further mixed 99 with a code generated by the pseudo-noise (PN) code generator 86 operating at a clock rate determined by code clock 85. The transmitter 88 transmits the twice modulated signal via antenna 90. The transmitted signal is received by receiver 94 via antenna 92. The received signal is correlated with a PN code generated by PN code generator 106 at mixer 96. A signal passing correlation is then demodulated by mixing the correlated signal with a local carrier recovery 104 in mixer 100. The transmitted data and clock rate are recovered in data processing 102 and 108, respectively. The recovered data 110 is passed to the remainder of the system for further processing while the recovered clock is used to drive the PN code generator 106 and the data processing 102 after being reduced by spreading gain G 112. The spreading gain is determined by the PN code rate Rc divided by the message rate Rb. The time duration of Rc is 1/Rc=Tc and is referred to as a chip. In general, when changing from a low-rate PN code to a higher rate PN code, where Tclow and Tchigh are the chip times, respectively, the spreading gain is multiplied by the ratio Tclow/Tchigh. The average time that is required to complete a search for PN timing is k/Rb per chip of uncertainty, where k is some constant of proportionality based on the search technique. Hence, if we do the initial search using a shorter low-rate PN and then synchronously switch to a higher-rate PN the average search time can be reduced by as much as Tclow/Tchigh.